The invention relates to refining of lignocellulosic fibrous material and particularly to thermomechanical pulping (TMP) and other mechanical refining processes.
TMP processes have conventionally refined fibrous material at high consistencies, typically having consistencies of 20 percent (20%) or more fiber by weight of the pulp suspension passing through the refiner. At high consistency levels, the pulp suspension is a fibrous mass and is transported by a pressurized blowline or screw conveyor which can handle such masses. In contrast, pulp suspensions at lower consistency levels flow as a liquid slurry that can be moved by pumps.
Mechanically refining pulp at a high consistency requires a large amount of energy that is expended primarily in frictional heat losses associated with viscoelastic deformations of the pulp in the refining zone. These frictional heat losses result in a large amount of energy that is not applied directly to refining pulp. Refining pulp is the separation (defibrate) and development (fibrillate) of the wood fibers. Typically less than 10% to 15% of the electric energy applied in a high consistency TMP refiner is directly applied to refining the pulp. There is a long felt need to increase the energy efficiency of a TMP refiner.
To address the need for lower electric energy consumption, TMP mills are searching for ways of displacing energy-intensive high consistency refining (HCR) with less energy intensive refining processes. Over the last ten to fifteen years many TMP mills have installed a single low consistency refining (LCR) stage directly following a mainline high consistency refining (HCR) stage. In most of these mill applications, the low consistency refiner (LCR) applies a specific energy less than 150 kWh/ODMT (kilowatt hours per oven dried metric ton) and displaces less than 100 ml (milliliters) of freeness.
Because low consistency refiners apply energy to a fluid pulp slurry, they tend to operate at significantly higher refining intensities than do high consistency refiners. However, the high refining intensities and fluid medium limits the total energy that can be applied in the refining zone of a LCR. Further, low consistency refining tends to produce pulp having limited freeness reduction. The limited displacement of freeness arises from excessive shearing of fibers and loss in pulp strength due to a narrow plate gap and a high energy load in a single stage of low consistency refining. Multiple stages of low consistency refining have been proposed. However, there is a practical limit to the number of LCR stages due to the inherent shearing of less developed (high freeness) mechanical pulp fibers in low consistency refiners.
Entailing fiber pretreatments to increase fiber flexibility and resistance to shearing resulted in a displacement of approximately 400 mL of high consistency refining with multiple stages of low consistency refining and energy savings of more than 30% as compared to conventionally produced thermomechanical (TMP) pulps. These entailing pretreatment methods included partial wood fiber defibration in a pressurized chip press (such as described in U.S. Pat. No. 6,899,791) followed by gentle fiber separation in a high consistency refiner (such as described in U.S. Pat. No. 7,300,541), chemical treatment, and high-pressure/high-intensity primary refining (such as described in U.S. Pat. No. 5,776,305, and U.S. Pat. No. 6,165,317). These pretreatments help improve fiber development and minimize fiber damage when low consistency refining across such a large span of freeness.
Despite continued advances in thermomechanical pulping there remain several long felt needs including: i) improving pulp quality development; ii) developing less-energy intensive pump-through refiners, and iii) reducing the complexity and cost of mechanical equipment in TMP systems.